1. Field of the Invention
This invention relates to a thin film transistor substrate applied to a display device, and more particularly to a thin film transistor substrate of fringe field switching type that has a simplified fabricating process.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) uses an electric field to control light transmission of a liquid crystal having a dielectric anisotropy to thereby display a picture. To this end, the LCD includes a liquid crystal display panel for displaying a picture through a liquid crystal cell matrix, and a driving circuit for driving the liquid crystal display panel.
Referring to FIG. 1, a related art liquid crystal display panel includes a color filter substrate 10 and a thin film transistor substrate 20 that are joined to each other with a liquid crystal 24 between them.
The color filter substrate 10 includes a black matrix 4, a color filter 6 and a common electrode 8 that are sequentially provided on an upper glass substrate 2. The black matrix 4 is provided in a matrix configuration on the upper glass substrate 2. The black matrix 4 divides an area of the upper glass substrate 2 into a plurality of cell areas to be provided with the color filter 6, and the black matrix 4 prevents a light interference between adjacent cells as well as external light reflection. The color filter 6 is provided in the cell area divided by the black matrix 4 in such a manner as to be divided into red (R), green (G) and blue (B) regions, thereby transmitting red, green and blue light. The common electrode 8 is formed from a transparent conductive layer entirely coated onto the color filter 6, and the common electrode 8 supplies a common voltage Vcom that serves as a reference voltage for driving of the liquid crystal 24. Further, an over-coated layer (not shown) for smoothing the color filter 6 may be provided between the color filter 6 and the common electrode 8.
The thin film transistor substrate 20 includes a thin film transistor 18 and a pixel electrode 22 provided for each cell area defined by a crossing of a gate line 14 and a data line 16 on a lower glass substrate 12. The thin film transistor 18 applies a data signal from the data line 16 to the pixel electrode 22 in response to a gate signal from the gate line 14. The pixel electrode 22, which is formed from a transparent conductive layer, supplies a data signal from the thin film transistor 18 to drive the liquid crystal 24.
Molecules of liquid crystal 24 having a dielectric anisotropy are rotated in accordance with an electric field generated by a data signal voltage between the pixel electrode 22 and a common voltage Vcom of the common electrode 8 to control light transmittance, thereby implementing a gray scale level.
Further, the liquid crystal display panel includes a spacer (not shown) for maintaining a constant cell gap between the color filter substrate 10 and the thin film transistor substrate 20.
In such a liquid crystal display panel, the color filter substrate 10 and the thin film transistor substrate 20 are formed by a plurality of mask processes. Herein, one mask process includes many sub-processes such as thin film deposition (coating), cleaning, photolithography, etching, photo-resist stripping and inspection processes.
Particularly, since the thin film transistor substrate includes semiconductor materials and requires a plurality of mask processes, its fabrication is complicated, which is a major factor in the manufacturing costs of the liquid crystal display panel. Therefore, reducing the number of mask process in fabricating the thin film transistor substrate is a key strategy in reducing manufacturing costs.
Liquid crystal display panels are generally classified into a vertical electric field type and a horizontal electric field type, depending upon the direction of the electric field driving the liquid crystal.
An example of the vertical electric field type is a twisted nematic (TN) mode liquid crystal display, in which a vertical electric field formed between a pixel electrode and a common electrode is arranged in opposition to each other on the upper and lower substrate. The vertical electric field type of liquid crystal display has an advantage of a large aperture ratio while having a drawback of a narrow viewing angle of about 90°.
An example of the horizontal electric field type is generated is an in plane switch (IPS) mode liquid crystal display, in which a horizontal electric field is generated between the pixel electrode and the common electrode arranged in parallel to each other on the lower substrate. A liquid crystal display of the horizontal electric field type has an advantage of a wide viewing angle about 160°, but has a disadvantage of low aperture ratio and transmittance.
In order to overcome the disadvantage of the liquid crystal display panel of horizontal electric field type, a liquid crystal display panel of fringe field switching (FFS) type has been developed, which operates by a fringe field. The FFS-type liquid crystal display panel includes a common electrode and a pixel electrode having an insulating film between them at each pixel area. Further, the fringe field allows all of liquid crystal molecules formed between the upper and lower substrates to be operated at each pixel area to thereby improve the aperture ratio and transmittance.
However, since the thin film transistor substrate included in the FFS-type liquid crystal display panel requires a plurality of mask processes as well as a semiconductor process, it has the disadvantage of a complicated fabricating process.